In the case of a watch which includes a seconds hand driven by the stepping motor and in which correction is mechanically obtained, it is sufficient to provide a motor which makes a number of steps per second equal to the number which must be made by the seconds hand in order to pass from one division to the following. This number of steps is most often equal to one and sometimes to two or to six. This signifies that the motor need only make a maximum of six steps per second which will not cause difficulties.
On the other hand, a problem arises when in a watch even without seconds hand, time setting corections or changes of the time zone are desired through direct control of the motor. In this case it is necessary to be able to have the motor operate at a frequency greatly increased over 6 Hz, i.e. six steps per second. Presently, with stepping motors of the Lavet type for instance, controlled in the standard manner, it is almost impossible to go beyond 64 Hz or even to guarantee a proper operation of the motor at a frequency greater than 50 Hz. In the case of a watch provided with minutes and hours hands in which the motor steps twice in order that the minutes hand advance through a division, should one wish to bring about a change in the time zone of six hours corresponding to the longest correction necessary if matters are arranged so that the motor may turn in both senses by using for instance the solution found in U.S. Pat. No. 4,112,671, for the control function, it is seen that even with a frequency of 64 Hz it will require more than eleven seconds in order to bring about this correction only. Clearly such is acceptable only with difficulty.
As to electronic time setting in a watch with a seconds hand, such can only be envisaged if one has available two motors for driving the hands, for example one for the seconds hand and one for the minutes and hours hand, or one for the seconds and minutes hands and another for the hours hand. The employment of a single motor would lead to exorbitant correction times.
Swiss patent application No. 5384/82 (corresponding to U.S. Pat. No. 4,550,279), gives an interesting solution which enables driving Lavet type motor in forward and backward operation at a speed greatly increased over 50 or 64 steps per second. This solution comprises the initial application of a starting pulse to the motor in forward or in backward operation to bring about the starting of the motor in the desired sense of rotation, thereafter a succession of simple maintenance pulses of alternating polarity and of duration shorter than that of the starting pulse, and finally simple stopping pulse of opposite polarity to that of the last maintenance pulse and longer than the latter. In the case of forward operation the starting pulse is a simple pulse while for the backward operation such starting pulse is formed from three basic pulses of alternating polarity, the polarity of the first pulse depending from the rest position initially occupied by the rotor. The first basic pulse has as function to urge the rotor in the forward direction in a manner such that it acquires a sufficient energy to be able to make the first step in backward operation under the effect of the two following pulses.
The instants of application of the maintenance and stopping pulses relative to the beginning of the starting pulse may either be determined in advance taking into account the motor characteristics or fixed through the detection of a parameter representative of the movement of the rotor, as for instance the voltage induced by the latter in the motor winding.
This manner of controlling a classic-type Lavet motor enables the employment for the forward operation as well as for the backward operation of a frequency very close to the theoretical synchronous frequency which is on the order of 200 Hz and leads thus to reasonable correction times for a watch indicating only minutes and hours, but still much too long for a watch having a seconds hand. Moreover, it requires a control circuit which is relatively complex.
This invention has as its principal purpose the furnishing of another solution to enable the operation of a stepping motor at a speed substantially greater than that attained up to the present time, at least in the horological domain.
A further purpose of the invention is to provide a solution which, employed in combination with known control methods for stepping motors, enables the attaining of speeds much greater than 200 steps per second.
Additionally, this solution must be adaptable for use with different types of motors, in particular those which are actually employed in the manufacture of watches.